1. Field of the Invention
The present invention relates to a rotary hearth furnace for manufacturing reduced iron through reduction of a raw material for reduced iron mainly composed of carbonaceous reducing material and iron oxide, and a screw thereof for discharging reduced iron on a hearth out of the rotary hearth furnace from an discharge port.
2. Description of the Related Art
As well known in the art, a rotary hearth furnace is used for manufacturing reduced iron through reduction of a raw material for reduced iron mainly composed of carbonaceous reducing material and iron oxide.
Such a rotary hearth furnace has a screw for discharging reduced iron on a hearth out of the rotary hearth furnace from an discharge port installed at an outer periphery side, that is, at a high-speed side on the furnace rotating about a vertical shaft. The durability of the reduced iron discharging screw is essential in enhancing the operation rate of the rotary hearth furnace or the productivity of reduced iron so that various means have been proposed up to now to enhance the durability of the reduced iron discharging screw.
According to an example of the reduced iron discharging screw, a cooling water channel is installed within a screw shaft or a rotary shaft, in which cooling water flowing through the cooling water channel lowers temperature to ensure the strength of the screw shaft, and a hollow portion is provided in a screw blade or a spiral blade, in which cooling water flowing through the hollow portion lowers temperature to ensure the hardness of the screw blade so that the wear resistance of the screw blade can be improved.
Further, according to another example of the reduced iron discharging screw, a cooling water channel is installed within a screw shaft or a rotary shaft of the reduced iron discharging screw, in which cooling water flowing through the cooling water channel lowers temperature to ensure the strength of the screw shaft thereby improving the durability of the screw shaft while two screw blades or spiral blades are joined together at a portion subjected to especially heavy abrasion to prolong the lifetime of the screw blades. Also, in order to further enhance the wear resistance of leading ends of the screw blades, an Inconel alloy (nickel 55% and chromium 45%) is built up via welding at both sides of the leading ends of the screw blades.
However, even if the strength of the screw shaft of the reduced iron discharging screw is enhanced and the screw blade has prolonged lifetime through improvement of the wear resistance of the screw blade and increase of wearing region, all kinds of reduced iron discharging screws require maintenance processes to detach the screw from the rotary hearth furnace for repair and to assemble the same to the rotary hearth furnace after repair. Such maintenance processes of the reduced iron discharging screw are carried out after stopping the operation of the rotary hearth furnace and the temperature within the rotary hearth furnace is lowered under a temperature which is suitable for the maintenance processes. In the maintenance processes, the reduced iron discharging screw is separated from a mooring equipment and a coupling, and then detached from the upper part through a furnace body cover of the rotary hearth furnace for repair, and assembled to the rotary hearth furnace from the upper part after repair.
In the reduced iron discharging screw of the prior art, corrosive gas generated from the raw material for reduced iron coheres to the surface of the screw shaft or the surface of the screw blade which is cooled through flowing of cooling water. Therefore, cohered corrosive gas causes cold corrosion to the surface of the screw shaft or the surface of the screw blade thereby decreasing the lifetime thereof Even in the case iron ores are reduced, sulfur included in coal mixed into staple pellets as a reducing agent generates SOX, which causes corrosion to the surface of the screw shaft or the surface of the screw blade thereby decreasing the lifetime thereof as the raw material for reduced iron.
In addition, in the prior art, the screw shaft using carbon steel has a service life of 4 to 10 months until cooling water leaks, and the screw blade has the lifetime of about 5 months. In other words, it can be hardly considered that any of the screw shaft or the screw blade has a sufficient service life and the maintenance operation of the reduced iron discharging screw should be performed frequently so that the operating ratio of the rotary hearth furnace cannot be enhanced Further, even if the screw blade is covered with a hard facing layer, which is constituted through hard facing of an Inconel alloy at both sides of the leading end and a leading portion of the screw blade by welding, sides of the screw blade tend to have welding defects such as an undercut between a base metal and a hard facing portion so that the screw blade may be damaged by a notch effect and accordingly the maintenance operation may be necessary even if other portions are sufficiently available.
In the reduced iron discharging screw having two screw blades joined together, the screw blades shaped as spiral blades are joined together at regions subjected to heavy abrasion to increase thickness thereby prolonging the lifetime of the screw blades. However, in this case, it is difficult to manufacture the two screw blades in high precision and the cost of the reduced iron discharging screw is inevitably increased because the screw blades are formed in three-dimensional shape.
Further, in the maintenance operation of the reduced iron discharging screw, since the cover of the furnace body should be detached at least as wide as the projected area of the reduced iron discharging screw and the cover of the furnace body is large, a radiation scheme is required in a large scale. Also, in regard to layout of the equipments, the equipments such as staple pins are frequently arranged at the upper position of the reduced iron discharging screw so that there is a problem that time and endeavor are required in a great amount to detach the reduced iron discharging screw according to arrangement among the equipments.
Accordingly, it is an object of the invention to provide a rotary hearth furnace for enabling a reduced iron discharging screw to be easily detached and assembled, and a reduced iron discharging screw of a rotary hearth furnace excellent in durability.
The present invention has been accomplished in view of the aforementioned circumstances, and to solve the foregoing objects, it is provided a rotary hearth furnace including a reduced iron discharging screw, which is rotationally supported via an axial end of a rotary shaft by a pair of supporting devices with the axial end of said rotary shaft passing through-holes formed in both side walls at one and the other sides of a furnace body and has a spiral blade at the outer periphery side of said rotary shaft for discharging reduced iron on a hearth out of the furnace body, from an discharge port formed at an outer periphery side of the rotating hearth, wherein the through-holes in said both side walls are set in a size for allowing the spiral blade of the reduced iron discharging screw to pass the same, and closed by a hatch member externally mounted to the axial end of said rotary shaft in a removable fashion; wherein the inner screw supporting device including a screw supporting metal member capable of reciprocating with a leading end detachably connected to the axial end of said rotary shaft, and a metal member dragging means for dragging the screw supporting metal member out of the furnace body is provided in the outside at an inner periphery side of said hearth; and wherein the outer screw supporting device including screw dragging means for removing said reduced iron discharging screw out of the furnace body, and a screw supporting bogie capable of reciprocating for supporting the removed reduced iron discharging screw is provided in the outside at an outer periphery side of said hearth.
To solve the foregoing objects, there is further provided a reduced iron discharging screw of a rotary hearth furnace for discharging reduced iron on a hearth out of a furnace body from an discharge port formed at an outer periphery side, which has a spiral blade at the outer periphery of a rotary shaft with the inside being cooled through flowing of cooling water; wherein a corrosion protective refractory layer is formed at the outer periphery of said rotary shaft.
To solve the foregoing objects, there is further provided a reduced iron discharging screw, which further comprises an elongated groove thinner than the thickness width of the spiral blade at the leading end of said spiral blade, wherein the elongated groove is filled with a hard facing layer.
To solve the foregoing objects, there is further provided a reduced iron discharging screw wherein said spiral blade has threads in a greater number at an outer periphery side of the hearth than at an inner periphery side of said hearth.
To solve the foregoing objects, there is further provided a reduced iron discharging screw wherein the axial end of said rotary shaft is supported via buffer members to be elevated and maintained at a certain height by a supporting device.